The present invention relates generally to a measuring method and apparatus, and, more particularly, to a measuring method and apparatus for measuring a wavefront aberration of a projection optical system that transfers a pattern of a mask onto an object using Point Diffraction Interferometry (PDI) or Line Diffraction Interferometry (LDI), and an exposure method and apparatus using the same. The measuring method and apparatus of the present invention is suitable, for example, to measure a projection optical system used for the exposure apparatus using EUV (Extreme Ultraviolet) light.
When semiconductor devices, for example, are manufactured in a photolithography process, an exposure apparatus that transfers a pattern formed on a mask onto an object is used. It is demanded that the exposure apparatus accurately transfer a pattern on a reticle onto an object to be exposed with a predetermined magnification, and thus, it is important that the exposure apparatus uses a projection optical system with good imaging performance that suppresses an aberration. Especially, because finer processing to a semiconductor device has been demanded in recent years, the transfer pattern has become sensitive to an aberration of the optical system. Therefore, there is a demand for highly accurate measurements of a wavefront aberration of a projection optical system.
The PDI using a pinhole is conventionally known for an apparatus that accurately measures a wavefront aberration of a projection optical system (for example, see Japanese Patent Application Publication No. 57-64139, U.S. Pat. No. 5,835,217 and Daniel Malacara, “Optical Shop Testing”, John Wiley & Sons, Inc., 231 (1978)). Moreover, not only is the PDI technique known, but also, the LDI is known (for example, see Japanese Patent Application Publication No. 2000-097666).
However, the inventor of this patent application has discovered that conventional PDI and LDI techniques have the following two problems against a wavefront aberration measurement with high accuracy.
The first problem is that a contamination often influences the pinhole and slit that generate a reference wave. For instance, the PDI technique generates a spherical wave by using the pinhole as the reference wave. To form an ideal spherical wave, a diameter of the pinhole is decided by a wavelength of the measuring light and by a diffraction limit of the ideal spherical wave given by the numerical aperture (NA) of a target optical system, which is given by λ/2NA. When the PDI measurement uses EUV light, it is necessary to reduce the pinhole diameter to about 30 to about 50 nm. Such a minute pinhole is subject to contamination. The LDI technique can generate a similar problem. For instance, when the EUV light is used as the measuring light, a hydrocarbon component included in the residual gas in the vacuum, for example, produces carbon as a result of chemical reactions with the EUV light, and clogs the pinhole. The clogged pinhole reduces the contrast and causes an interference fringe to disappear. The pinhole deforms during clogging, and a point diffracted light as a reference wavefront shifts from the spherical wave by a change of a pinhole shape. This causes an erroneous detection in a wavefront analysis of the projection optical system.
The second problem is that the pinhole shifts from a condensing point of light. (This shift will be called “drift” in this specification.) Typical drift examples include time-sequential drifting of the pinhole mask and drifting of the incident light. The former is generated by the measuring light, and the pinhole mask in which the pinhole thermally deforms. The latter occurs when a light source position changes with time due to heat, for example. In any case, the contrast of the interference fringe decreases, because pinhole transmitting light decreases, and causes the above problem. A similar problem can occur in the slit for the LDI.